A magnetic field sensor for measuring magnetic fields is known from EP 1 182 461. The magnetic field sensor is suitable for determining the direction of a two-dimensional magnetic field. The magnetic field sensor comprises a magnetic field concentrator with a flat shape and two sensors that comprise at least one Hall element, whereby the Hall elements are arranged in the area of the edge of the magnetic field concentrator. The first sensor measures a first component of the magnetic field and the second sensor measures a second component of the magnetic field. The direction of the magnetic field can therefore be determined from the signals of the two sensors.
A further magnetic field sensor for determining the direction of a magnetic field is known from EP 1 052 519. The magnetic field sensor comprises a ferromagnetic core in the form of a cross, an excitation coil for periodically saturating the ferromagnetic core, and read-out coils. The magnetic field sensor is operated as a flux gate sensor. The disadvantage with such a sensor is that a relatively high current is necessary for the magnetic saturation of the ferromagnetic core. Such a magnetic field sensor is therefore not suitable for applications with battery operation.
A further magnetic field sensor for determining the strength of a magnetic field is known from GB 2315870. The magnetic field sensor comprises a ferromagnetic core in the form of a ring, an excitation coil in order to periodically saturate the ferromagnetic core and read-out coils. Furthermore, in one design type, the sensor comprises additional ferromagnetic cores that work as external magnetic field concentrators. To reduce a possible residual magnetism of these additional cores, additional coils are present to which current is periodically applied in order to demagnetize the additional cores.
The object of the invention is to develop a magnetic field sensor with which magnetic fields can be measured the strength of which only amounts to some nT to mT even when power is supplied by a battery without the battery having to be frequently changed.
Investigations have revealed that a significant problem exists in that the ferromagnetic core serving as magnetic field concentrator can be magnetized or the magnetization can be reversed to such an extent by an external, for example temporarily occurring magnetic interference field, that the ferromagnetic core becomes a magnetic dipole that produces a signal in the read-out coils. Here, the invention provides a remedy in that the ferromagnetic core is brought into a state of predetermined magnetization by applying a magnetic field at selected times. A coil is provided for producing the magnetic field. The magnetic field produced by the current flowing through the coil must be large enough that the magnetic reversal of the ferromagnetic core caused by the interference field can be cancelled. In doing so, the necessary current intensity is dependent on the magnetization curve of the ferromagnetic core.
A magnetic field sensor in accordance with the invention for the measurement of at least one component of a magnetic field comprises a ring-shaped ferromagnetic core that serves as a magnetic field concentrator, an excitation coil and a read-out sensor. The read-out sensor comprises at least one, preferably two sensors arranged in the vicinity of the outer edge of the ferromagnetic core and measures the at least one component of the magnetic field. On operation of the magnetic field sensor, a current is temporarily applied to the excitation coil at selected times in order to bring the ferromagnetic core into a state of predetermined magnetization in which the magnetization of the ferromagnetic core produces no signal in the read-out sensor. The current flowing through the excitation coil must be large enough so that the magnetic field produced by the current in the ferromagnetic core achieves at least the coercive field strength given by the material of the ferromagnetic core. Preferably, the current is selected so high that the magnetic field produced by it is two to three times greater than the coercive field strength. If the material has a so-called hard and a soft magnetic axis, then the greater coercive field strength of the hard magnetic axis has to be selected. With this process, the ring-shaped ferromagnetic core is magnetized for example in such a way that the field lines within the core run as closed field lines in tangential direction. This magnetization is called circular magnetization. By means of this predetermined magnetization, the problem of the previously mentioned residual magnetization that leads to errors is solved.
This process of bringing the ferromagnetic core into a state of predetermined magnetization is preferably carried out before an actual measurement of the external magnetic field. It can however be carried out periodically or at any other time. The ferromagnetic core is thus magnetized with a predetermined magnetization and this predetermined magnetization is refreshed or restored at specific times.
In order to bring the ferromagnetic core into the desired state of magnetization, a certain amount of magnetization energy is necessary. The required magnetization energy is, on the one hand proportional to the volume V of the ferromagnetic core and, on the other hand to the magnetic product B×H that is dependent on the hysteresis curve of the material used. In order to get the smallest possible magnetic product, a soft magnetic material, eg, Vitrovac 6025Z is selected as the material for the ferromagnetic core. The volume to be magnetized is determined by the geometry of the ferromagnetic core. Because the achievable magnetic amplification is mainly determined by the diameter of the ring-shaped ferromagnetic core, the width and thickness of the ring are selected as small as possible. For the above-mentioned material Vitrovac 6025Z, with a ring diameter of 1 mm for example, a width of 20_m and a height of 10_m are selected. The width therefore amounts to only two percent of the diameter of the ring. It makes sense when the width and height of the ring are as small as is possible as the used technology allows it. When using other technologies such as for example attaching the core to a semiconductor chip by means of electrolysis or sputtering, the thickness of the core can be reduced to one micrometer or less.
A further advantage of reducing the volume of the core exists in that the build up of the premagnetization by means of a magnetic field produced by the coil is itself less inhibited by eddy currents produced in the core. In this way, the current pulse for magnetization can be shorter and the total required energy reduced. A sensor optimised in this way is also suitable for applications with little admissible energy consumption such as a watch for example.
The magnetic field sensor can be used for example in order to measure the strength of a weak magnetic field the direction of which does not change. Such a magnetic field sensor can also be used as a current or energy sensor whereby it measures the strength of a magnetic field produced by a conductor with current flowing through it. In addition, the sensor can have a second read-out sensor in order to measure a second component of an external magnetic field. When two components of the external magnetic field are measured, then its direction can also be determined from them. Such a magnetic field sensor can therefore also be used as a compass.
In the following, three embodiments of the invention are explained in more detail based on the drawing.